Optical amplifiers for broadband optical signals exhibit a wavelength dependency of the gain which is not completely remedied by the smoothing filters normally used. In WDM or DWDM (WDM=Wavelength Division Multiplex; DWDM=Dense Wavelength Division Multiplex) transmission technology the optical signal consists of a number of channels featuring different wavelengths, with the spacing between the wavelengths nowadays able to be below 100 GHz. The wavelength dependency of the gain of amplifiers means that power differences accumulate between the individual channels when an optical signal passes through them so that the channels possess greatly differing optical signal-to-noise ratios OSNR and powers at the receivers.
In point-to-point-data connections therefore a known method referred to as pre-emphasis is used for equalizing the optical signal-to-noise OSNR values at least at the end of the link, said method having been described in A. R. Chraplyly, J. A. Nagel and R. W. Tkach: “Equalization in Amplifier WDM Lightwave Transmission Systems”, IEEE Photonics Technology Letters, Vol. 4, No. 8, 1992, pp. 920-922. In this method the transmit-side channel powers are adjusted on the basis of an OSNR distribution measured at the link end in an iterative process until the same optical signal-to-noise ratio OSNR values are produced at the link end for all channels.
Frequently for determining the signal-to-noise ratios OSNR the Amplified Spontaneous Emission (ASE) between the channels is measured and from this the noise power overlaid on the channels is calculated by interpolation. This is however no longer possible if the amplified spontaneous emission ASE between the channels is attenuated by optical components. This is the case for example if further modules such as add-drop modules or interleaver filters are connected into the transmission link.